Probably you have couple appliances that are reliant on constant power supply. It can be anything like fridge, water pump, server, router. If these things are next to you you can react instantly if power supply fails. But what if this happens in remote area. If you have fridge somewhere else and power fails, in time you can get loads of crap. In order to avoid such situations Medhi started Arduino power failure alarm project, where circuit would detect when power fails and send short message to owner so he could take actions immediately and save a day.
Alarm device is based on Arduino. In order to send SMS, he used old Nokia3310 phone, which has plenty of info on interfacing using FBUS commands. Next thing here was to detect power failure. This is implemented using Arduino power supply with backup 9V battery. Arduino simply senses incoming voltage from supply adapter and if it fails, then it switches to backup battery. In order to have enough power to switch between supplies there is a large cap that prevents from resetting the board. I don’t know if this is a good way of doing this, but if it works, why not.